Consider the following pairs of ions

$\mathrm{A}) {\mathrm{Sc}}^{+3} \mathrm{and} {\mathrm{Ti}}^{+4}$

$\mathrm{B}) {\mathrm{Mn}}^{+2} \mathrm{and} {\mathrm{Fe}}^{+2}$

$\mathrm{C}) {\mathrm{Fe}}^{+2} \mathrm{and} {\mathrm{Co}}^{+3}$

$\mathrm{D}) {\mathrm{Cu}}^{+} \mathrm{and} {\mathrm{Zn}}^{+2}$

Among these pairs of ions, isoelectronic pairs would include

 

Consider the electronic configuration of each species in all pairs.

$\mathrm{A}) {\mathrm{Sc}}^{+3}\left(\mathrm{Z}=21\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^6 \mathrm{or} \left[\mathrm{Ar}\right]$${\mathrm{Ti}}^{+4}\left(\mathrm{Z}=22\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^6 \mathrm{or} \left[\mathrm{Ar}\right]$$\mathrm{B}) {\mathrm{Mn}}^{+2}\left(\mathrm{Z}=25\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^5 \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^5$${\mathrm{Fe}}^{+2}\left(\mathrm{Z}=26\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6 \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^6$$\mathrm{C}) {\mathrm{Fe}}^{+2}\left(\mathrm{Z}=26\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6 \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^6$${\mathrm{Co}}^{+3}\left(\mathrm{Z}=27\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6 \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^6$

$\mathrm{D}) {\mathrm{Cu}}^{+1}\left(\mathrm{Z}=29\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^{10} \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}$${\mathrm{Zn}}^{+2}\left(\mathrm{Z}=30\right)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^{10} \mathrm{or} \left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}$Therefore, in the above all only A, C and D pairs have the same number of electrons.